The present invention relates to a method for calibrating one or more patient blood volume status indicators derived from patient blood pressure data for changes in the latter arising from changes occurring in the mechanical ventilation of the patient.
In the ventilation of a patient with a mechanical ventilator, positive pressure breathing gases are supplied to the patient by the ventilator during inspiration. Expiration usually occurs from the natural relaxation action of the patient's thorax. The increase in intrathoracic pressure resulting from the expansion of the lungs by the positive pressure breathing gases causes changes in blood pressure measurements obtained from the patient. In general, an inspiratory intrathoracic pressure increase results in a decrease in arterial systolic and diastolic pressures that lags behind the intrathoracic pressure increase. This change is often termed the “respiratory swing,” and is more pronounced under conditions of reduced blood volume in a patient, termed “hypovolemia” so that the extent of the changes in the cardiological data may be used to detect the state of, or changes in, patient blood volume. To this end, indicators such as the systolic blood pressure variation (SPV), the delta down component of SPV, or the variation in pulse pressure are employed. The “delta down” component of SPV is the difference between a minimal systolic value obtained during breathing and the minimal value obtained during a period of apnea (non-breathing state). For SPV, this indicator increases following a reduction in patient blood volume (hypovolemia) and decreases following an increase in patient blood volume (hypervolemia). U.S. Pat. No. 6,585,658 shows a method and system for automating patient blood volume status indicators obtained from cardiological patient data.
It is also known that when a patient is mechanically ventilated, changes in ventilation parameters or patient lung conditions will change the blood pressure data obtained from the patient causing changes to occur in the blood volume status indicator(s) derived from the blood pressure data. See, for example, The Effect of Tidal Volume and Intravascular Volume State on Systolic Pressure Variation in Dogs, R. Szold, et al., Intensive Care Med (1989), Vol. 15, pages 368 et seq. describing the effect of variation in tidal volume VT delivered by a ventilator on SPV and other patient blood volume status indicators.
In the past, it was common practice to carry out the mechanical ventilation using generally constant operating parameters for the ventilator. Depending on the patient treatment regimen, ventilator operating parameters typically include such parameters as the aforementioned tidal volume (VT), as well as inspiratory pressure level (Pi), the ratio of the inspiratory time period to the expiratory time period (I:E ratio), the breath rate (BR), and the positive pressure maintained at the end of expiration (PEEP). As the ventilator settings were not changed, or only infrequently changed, in the course of a patient treatment, or among different patients, there was little loss in the clinical usefulness of the blood volume status indicator(s) obtained from the blood pressure data during ventilation of the patient. If ventilation settings were changed, clinicians would often attempt to estimate the effects of the changes on the blood volume status indicators in an effort to maintain the accuracy and usefulness of the indicators.
In more modern and sophisticated uses of mechanical ventilators, the operating parameters are more often varied over the course of a mechanical ventilation of a patient rather than remaining constant. This may be done, for example, in accordance with a sophisticated treatment regime for the patient, to promote weaning of the patient from the ventilator, or because conditions in the patient's lung, such as lung compliance, change. Lung compliance describes the relationship between the magnitude of a volume of breathing gas supplied to the patient's lungs and the magnitude of the resulting pressure produced in the lungs.
At present, however, as ventilator parameters or lung conditions change, resulting in changes in blood pressure data obtained from a patient, no effort is made to automatically calibrate or correct the blood pressure volume status indicators obtained from the blood pressure data. This means that, when such changes occur, an attending clinician will be provided with inaccurate information or information that is not on a uniform basis. As small changes in the indicators can represent significant changes in blood volume status, inaccurate information can mask important patient status changes and place the patient at risk.